The invention relates to a method of assigning an idle channel in which an idle channel is assigned in response to a link channel establishment request transmitted from a mobile terminal such as a portable telephone or a PHS (personal handyphone system) to a base station, and particularly to a method of assigning an idle channel to efficiently use frequencies.
FIG. 3 is a diagram showing an example of the configuration of a base station. As shown in the figure, the base station comprises an antenna 11, a selector switch 12, an amplifier 13, a mixer 14, an IF section (an intermediate frequency section) 15, a modem section 16, an interface 17, a synthesizer 18, a mixer 19, and an amplifier 20.
The base station receives a signal transmitted from a mobile terminal in its own service area, via the antenna 11. The received signal is supplied to the amplifier 13 through the selector switch 12. The amplifier 13 amplifies the signal. The signal is then subjected to frequency conversion by an output of the synthesizer 18 in the mixer 14, and subjected to filter and amplification processings in the IF section 15. Thereafter, the signal is demodulated in the modem section 16 and then output to a line (a usual telephone line) via the interface 17.
A signal from the line is modulated in the modem section 16 and then subjected to filter and amplification processings in the IF section 15. Then, the signal is converted into a high-frequency signal in the mixer 19, amplified in the amplifier (power amplifier) 20, and transmitted to the mobile terminal from the antenna 11 through the selector switch 12.
The synthesizer 18 is an oscillator which generates a signal of a channel frequency (carrier). The output frequency of the synthesizer 18 is set by a channel set signal from the modem section 16, and control and message channels are established. When a mobile terminal is moved into a service area of a base station, the mobile terminal transmits a link channel establishment request to the base station. The base station assigns an appropriate channel selected from idle channels and exchanges information with the mobile terminal.
FIG. 4 is a diagram showing an example of service areas of base stations and channel assignment. In the illustrated example, base stations a, b, c, and d which are connected to a network communicate with mobile terminals in their service areas. First, a mobile terminal a transmits a link channel establishment request to the base station a. The base station a which receives the link channel establishment request informs the mobile terminal a of an information channel (a control channel) which is previously (or after the request) judged as an idle channel as a result of the carrier sensing operation (carrier search), so as to establish the link channel.
In the assignment of an idle channel, a channel having a receiving level equal to or higher than a detectable level, and having the lowest interference level (disturbance level) is selected so as to obtain good speech quality. In other words, a channel (carrier) having the highest DU ratio (Desired signal (signal receiving level) to Undesired signal (interference level) ratio) is selected. In the case where there are regulations prescribed by the laws or standards, a channel having a level equal to or higher than the prescribed level is selected.
In an example case, only two carriers, i.e., a carrier 1 and a carrier 2 are available, the carrier 1 is used by the base station c, and the base station a communicates with a mobile terminal in each area by using the carrier 2. In this case, when the base station a performs the carrier sensing operation, the interference level is increased because the carrier 1 is used by the base station c. The base station a selects the carrier 2 and assigns it to the mobile terminal a, so as to establish the link channel.
In the above-described example case, even when the carrier 1 has the available level for the base station a, the carrier 2 is assigned to the mobile terminal a in the service area a. Since the carrier 1 and the carrier 2 are used in the service areas adjacent to the base station b, therefore, the base station b has no available carriers. In such a case, there arises a problem in that it is necessary to provide another carrier (channel) and frequencies cannot be effectively used.